Photometric apparatus and method

ABSTRACT

A photometric analyzer and method are disclosed for analyzing particle containing samples. More particularly, a nephelometric apparatus and method are disclosed for analyzing bacteria samples. In the disclosed embodiment, the bacteria count of a bacteria sample is determined by photometrically comparting the sample to another sample having a known particle or bacteria concentration. Thus, in accordance with the invention, samples having a known bacteria count may be quickly and easily obtained from a &#34;go&#34; &#34;no-go&#34; test. The bacteria samples obtained according to the invention may be used as standardized bacteria concentrations for conducting Kirby-Bauer dilution tests.

BACKGROUND OF THE INVENTION

The present invention relates to the photometric analysis of samples,and more particularly to the nephelometric analysis of bacteria samples.

DESCRIPTION OF THE PRIOR ART

Photometric analysis apparatus and methods are known in which light isprojected towards the sample to be analyzed and the light emerging frompredetermined regions of the sample is detected by photoelectric meanswhile light emerging from the sample in regions other than thepredetermined regions is prevented from reaching the photoelectricmeans.

In Loeschcke et al. U.S. Pat. No. 2,769,365, two photoelectric cells areprovided, one for receiving the light emerging from a predeterminedregion of the sample corresponding to light diffracted and diffused bythe sample and the other for receiving undiffracted and undiffusedlight. A lens is used to collect the light projected by the light sourceand transmit the collected light towards the sample. Both cells arecoupled to a galvanometer which reads a null when both cells arereceiving the same amount of light. The undiffracted and undiffusedlight reaching the other photoelectric cell is adjusted to obtain a nullreading on the galvanometer. The adjustment is calibrated to read thenumber of particles in the sample.

In Coyne et al. U.S. Pat. No. 4,072,421, lenses are used to focus lightfrom a light source at an optical interaction station through whichparticles are passed and to focus the light scattered by certainparticles which are to be counted on a photodetector. The unscatteredlight passing through the optical interaction station and the light fromother particles is prevented from reaching the photodetector by a lightstop disposed on an optical axis extending between the photodetector andthe light source. A particle of the concerned type is counted each timescattered light is detected by the photodetector.

Kompelin U.S. Pat. No. 3,185,975 discloses a photoelectric smokedetector in which light from a light source is projected across arelatively large surface area and any light reflected by particles whichlie in an annular space in the large surface area are detected. A lightblock is interposed between the light source and a photocell on anoptical axis therebetween to prevent light, both reflected and direct,which is in a central region on the side of the light block facing thelight source from reaching the photocell.

Gibbs U.S. Pat. No. 3,549,893 discloses a photoelectric liquid levelsensor. Light is projected towards a chamber in which the liquid levelis to be sensed. A light responsive cell is disposed on the other sideof the chamber and a light baffle is disposed between the cell and thechamber. Light passing through an empty chamber diverges and isprevented from reaching the cell by the baffle because the cell isdisposed to be in an umbra produced by the baffle. When a liquid ispresent in the chamber, the light passing through the liquid isconverged and the light in a penumbra reaches the cell.

In performing Kirby-Bauer bacteria sensitivity tests, various drugs areintroduced into a bacteria sample and the reactions of the bacteria tothe drugs are tested. In order to run the tests, the bacteria sample isstandardized to have a predetermined bacteria count.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a simple andrelatively inexpensive photometric apparatus.

It is another object of the present invention to provide a photometricapparatus and method for comparing an unknown sample to a known sample.

It is still another object of the present invention to provide a simpleand inexpensive nephelometric apparatus and method for analyzingsamples.

It is also an object of the present invention to provide a nephelometricapparatus and method for comparing an unknown to a known sample.

It is a further object of the present invention to provide an apparatusand method for photometrically comparing an unknown sample to a knownsample on a "go", "no-go" test basis.

In accordance with the invention, nephelometric apparatus and a methodare provided for analyzing a fluid sample of particulate material. Abeam of light is projected about an optical axis towards the sample, thelight diverging from the optical axis. Light emerging from apredetermined region in the sample is permitted to reach photoelectricmeans spaced from the sample while light propagating along the opticalaxis and in a region extending radially about the axis is prevented fromreaching the photoelectric means. Transmission of the direct image of alight source means projecting the beam of light is also prevented fromreaching the photoelectric means. Accordingly, the light reaching thephotoelectric means will be proportional to the amount of lightscattered through an angular range (determined with respect to theoptical axis) by the particulate material in the sample. An electricalcharacteristic of the photoelectric means which is dependent upon thelight which reaches the photoelectric means may be used to determineselected characteristics of the sample, such as the concentration ofparticulate material in the sample.

More specifically, a sample is photometrically analyzed according to oneaspect of the invention by:

Projecting a beam of light about an optical axis towards the sample, theprojected light diverging from the optical axis; permitting lightemerging from the sample in a predetermined region spaced from anextending radially about the optical axis to be detected at a locationspaced from the sample; preventing light propagating along the opticalaxis and in an adjacent region extending radially about the axis frombeing detected at said location; and providing an indication of thecount of the sample proportional to the amount of light detected. Inaccordance with a preferred embodiment, light propagating along theoptical axis and in the adjacent region is prevented from being detectedafter such light emerges from the sample. This is accomplished by meansdisposed between the sample and photoelectric means which photoelectricmeans receive and detect the light in the predetermined region.

According to another aspect of the invention, an unknown sample iscompared to a known sample by projecting light having given opticalcharacteristics towards the known sample and detecting light emergingtherefrom; adjusting an indication of the intensity of the lightdetected to provide a first reading; projecting light with the givenoptical characteristics towards the unknown sample and obtaining asecond indication thereof; and comparing the first and secondindications.

In accordance with still another aspect of the invention, nephelometerapparatus is provided for analyzing a fluid sample of particulatematerial disposed in a container comprising:

light source means for projecting light about an optical axis, theprojected light diverging from the optical axis; photoelectric meansspaced from the light source and disposed adjacent said optical axis forreceiving light from the light source means, the electricalcharacteristics of the photoelectric means being dependent upon theintensity of light received by the photoelectric means; means forpositioning the container at a predetermined location intersecting thediverging projected light and spaced apart from the light source meansand the photoelectric means; first baffle means opaque to the light fromthe light source means disposed adjacent one of the opposite sides ofthe positioning means and intersecting said optical axis and extendinggenerally radially therefrom for preventing light in a firstpredetermined region extending radially about the optical axis andincluding any direct image of the light source means from reaching thephotoelectric means while permitting light emerging from the containerin the positioning means in a spaced predetermined region extendingradially beyond said first predetermined region to reach thephotoelectric means; and indicator means coupled to said photoelectricmeans for proportionally indicating the intensity of the light receivedby the photoelectric means in accordance with the electricalcharacteristics of the photoelectric means.

In a preferred embodiment, the light source means comprises a lightsource and a second baffle means opaque to the light source disposedintermediate the positioning means and the light source, the secondbaffle means having an orifice therein in alignment with the opticalaxis. The area of the orifice is substantially smaller than the area ofthe light source from which the light is projected and the orifice ispreferably beveled on the side thereof opposite to the light source.

In one embodiment, the first baffle means is disposed intermediate thepositioning means and the photoelectric means, whereby the light in thefirst predetermined region and including any optical image of the lightsource means emerging from the container is prevented from reaching thephotoelectric means. The first baffle means extends radially in onedirection to prevent all light emerging along said direction from thecontainer in the positioning means from reaching the photoelectric meanswhile permitting light in a radial portion of the second predeterminedregion to reach the photoelectric means. The positioning means isdisposed in the apparatus to receive a cylindrically-shaped samplecontainer having a cylinder axis disposed transverse to the opticalaxis, and the first baffle means comprises an elongated member disposedtransverse to the optical axis and the cylinder axis. The photoelectricmeans comprises a photoresistor and the indicator means comprises ameter electrically coupled with the photoresistor.

In accordance with another aspect of the invention, photometricapparatus is provided comprising: means for positioning a containerholding a sample to be analyzed; a source of light disposed to one sideof the positioning means for projecting light towards the positioningmeans to impinge on the container in the positioning means;photoelectric means disposed on a side of the positioning means opposedto said one side for receiving light emerging from the container in thepositioning means, the electrical characteristics of the photoelectricmeans being dependent on the intensity of the light received by thephotoelectric means; indicator means coupled to said photoelectric meansfor proportionally indicating the intensity of the light received by thephotoelectric means in accordance with the electrical characteristics ofthe photoelectric means; first adjustment means coupled to saidindicator means for adjusting the indication of the indicator means whenthe container contains a "zero" sample; second adjustment means coupledto said indicator means for adjusting the indication of the indicatormeans when the sample container contains a standard sample; wherebyafter the first and second adjustment means have been adjusted, acontainer holding an unknown sample when received in the positioningmeans can be calibrated against the standard sample when the indicatormeans has the same indication as that for the standard sample.

In accordance with a further aspect of the invention, photometricapparatus is provided comprising: a source of light for projecting lighttowards a sample to be analyzed; photoelectric means positioned toreceive light projected by the source emerging from the sample andhaving electrical characteristics dependent upon the light received bythe photoelectric means; DC power source means for providing DC power atan output thereof which is coupled to the photoelectric means; indicatormeans coupled to the photoelectric means for indicating an electricalcharacteristic of the photoelectric means; first adjustment meanscoupled between the photoelectric means and the DC power source meansfor adjusting the DC power provided to the photoelectric means; andsecond adjustment means including means for providing DC power to theindicator means at a level which is adjustable. In the preferredembodiment according to this aspect of the invention: the firstadjustment means comprises a resistive voltage divider coupled betweenthe output of the source of DC power and the photoelectric means; thephotometric means comprises a photoresistor; and the second adjustmentmeans comprises a resistive voltage divider coupled between the sourceof AC power and the indicator means and includes a potentiometer havingits fixed terminals coupled to the source of AC power and its adjustableterminal coupled to the indicator means through the rectifier meanswhich comprises a diode connected to provide a negative half-waverectified voltage to the indicator means, the DC power source meansproviding a positive DC voltage to the first adjustment means.

Standardized concentrations of bacteria samples are used to determinethe sensitivity of the bacteria to different drugs. For example, in theKirby-Bauer dilution tests, various drugs and various concentrationsthereof are introduced into a standardized bacteria sample to determinewhether the bacteria is sensitive or resistant to the drug and thedifferent concentrations thereof. In accordance with the Kirby-Bauertest, the minimum inhibitory concentration (MIC) of a drug for aparticular bacteria may be determined.

Bacteria samples used in Kirby-Bauer dilution tests are standardized tohave an actual bacteria count of from about 0.5×10⁸ to about 5×10⁸ perml depending upon the particular bacteria. For example, forstaphylococus bacteria, the standardized count is about 0.5×10⁸ per mland for pseudomonas bacteria the standardized count is about 5×10⁸ perml.

It has been found that bacteria samples having the same photometricreading taken according to the invention as a McFarland standardconcentration will have an actual bacteria count of from about 1 toabout 2×10⁸ per ml and are suitable as standards for conductingKirby-Bauer dilution tests. Thus, in accordance with the invention,different bacteria samples whose desired standardized concentrationvaries from about 0.5×10⁸ to about 5×10⁸ per ml, a range of ten for thedifferent bacteria counts, may be obtained with an actual count of fromabout 1 to about 2×10⁸ bacteria per ml, a range of 2, and may be used asstandard concentrations in Kirby-Bauer dilution tests.

For example, a staphylococcus stample standardized according to theinvention will have a count of from about 1×10⁸ per ml to about 2×10⁸per ml and a pseudomonas sample standardized according to the inventionwill also have a count of from about 1×10⁸ per ml to about 2×10⁸ per ml,both relatively close to the desired counts of 0.5×10⁸ per ml and 5×10⁸per ml, respectively.

According to this aspect of the invention, the bacteria count in abacteria sample is compared to the particle count in a McFarlandstandard concentration by: projecting a beam of light about an opticalaxis towards the McFarland standard concentration in a first containerat a predetermined location; detecting light emerging from the McFarlandstandard concentration in a predetermined annular region of the firstcontainer radially displaced from the optical axis; obtaining areference indication proportional to the light detected emerging fromthe McFarland standard concentration in the predetermined annularregion; projecting light in the beam and along the optical axis towardsa bacteria sample in a second container which is substantially identicalto the first container at the predetermined location; detecting lightemerging from the bacteria sample in the predetermined annular region;obtaining another indication proportional to the light detected emergingfrom the bacteria sample in the predetermined annular region andcomparing the other indication to the reference indication.

In accordance with a preferred embodiment of the invention, aphotometric apparatus and method are provided for standardizing abacteria sample quickly and relatively accurately and at a low cost.

These and other aspects of the present invention will be more apparentfrom the following description of the preferred embodiment whenconsidered with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likenumerals indicate similar parts and in which:

FIG. 1 is a perspective view of the apparatus according to theinvention;

FIG. 2 is a schematic view of the optical portion of the apparatus ofFIG. 1 depicting the light source, two light baffles, the sample to beanalyzed and photoelectric means;

FIG. 3 is a schematic view of the optical portion shown in FIG. 2 takenalong lines 3--3 of FIG. 2;

FIG. 4 is an enlarged schematic view of the optical portion shown inFIG. 2 taken along lines 4--4 of FIG. 2;

FIG. 5 is a schematic circuit diagram of the apparatus of FIG. 1; and

FIG. 6 is an enlarged schematic view, similar to FIG. 2, of anotherembodiment of the optical portion.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now more particularly to the drawings, a nephelometer 10 isillustrated for testing bacteria samples to determine whether thesamples contain a predetermined bacteria count and hence whether thesample is standardized. Accordingly, the apparatus is referred to as aStandardized Inoculum Reader.

To determine whether the bacteria samples contain the standardizedconcentration utilizing the Standardized Inoculum Reader 10 shown inFIG. 1, the Reader 10 is calibrated using a sample having a McFarlandstandard concentration so that the indicator 11 of the meter 12 has areading in the central area 14 of the meter face between the "+" area 16and the "-" area 18 and superposed with line 20. The McFarland standardconcentration is a solution of barium sulphate (BaSO₄) particles in aconcentration of about 1×10⁸ particles per ml. After the Reader 10 hasbeen calibrated for the McFarland standard concentration, bacteriasamples are tested against the McFarland standard concentration.Bacteria samples for which bacteria counts of from about 0.5×10⁸ toabout 5×10⁸ per ml are desired, when tested in the Reader 10 will give ameter reading in the area 14 corresponding to an actual bacteria countof from about 1×10⁸ per ml to about 2×10⁸ per ml.

Referring now to FIGS. 1-3, the Standard Inoculum Reader 10 includes anoptical section 22 which comprises a lamp 24, an opaque baffle 26 havingan orifice 28 therein, another opaque baffle 30 and a photoresistor 32.The bacteria sample to be analyzed is contained in a sample container 33in the form of a test tube which is inserted into a receptacle 34 toconduct a test. The optical section 22 is enclosed in an opaque housing35 to prevent stray light from reaching the sample container. Thereceptacle 34 includes a pressure sensitive switch 36 (FIG. 5) which isengaged by the bottom of the test tube 33 and which controlsillumination of the lamp 24. Depressing the container 33 activates theswitch to switch the lamp on, and releasing the container, deactivatesthe switch to switch the lamp off. The lamp 24 projects light towardsthe opaque baffle 26 about an optical axis 38 (FIG. 2) which passesthrough the orifice 28 in a diverging beam 39. The baffle 30 and thephotoresistor 32 are disposed along the optical axis 38 to one side ofthe container 33 while the lamp 24 and baffle 26 are disposed to theopposite side of the container 33. The photoresistor 32 is spaced fromthe receptacle 34. The space 41 (FIG. 4) between a container 33 in thereceptacle and the photoresistor is open except for the disposition ofthe baffle 30 therein.

The diameter of orifice 28 is about two mm while the face of lamp 24 isapproximately three cm in diameter. Accordingly, beam 39 diverges as itpropagates from orifice 28 through container 33. Orifice 38 is believedto assist in providing a diverging beam.

The particles suspended in the McFarland standard concentration scatterlight from beam 39 as it passes through container 33. For example, lightray 40 of beam 39 is intercepted by particles at 42 and is scatteredtoward photoresistor 32. By virtue of baffle 30, light within the regionbounded by the imaginary lines 43 and passing through the imaginaryregion shown at 46, which is transmitted directly (i.e. withoutscattering) from lamp 24 through container 33 toward photoresistor 32 isprevented from intercepting the latter. Correspondingly, light from beam39 scattered within that portion of container 33 opposite baffle 30 alsowill not reach photoresistor 32. Accordingly, photoresistor 32 onlydetects light scattered by particles suspended in the sample and fallingwithout that portion of container 33 which is not directly oppositebaffle 30.

An upper sleeve 45, opaque to light in beam 39, is disposed about theupper portion of container 33 and extends downwardly to a level abovethe baffle 30. Sleeve 45 serves to minimize errors due to meniscusreflection and scattering, and due to ambient light, and to define thelargest detectible scattering angle (measured with respect to opticalaxis 38) in the upper part of container 33. A lower sleeve 47, alsoopaque to light in beam 39, is disposed about the lower portion ofcontainer 33 and extends upwardly to a level below the baffle 30. Sleeve47 serves to minimize the effects of bottom focus and scattering, and todefine the largest detectible scattering angle in the lower part ofcontainer 33. The mean scattering angle is determined principally by thedistance of baffle 30 from container 33 and the distance of baffle 30from photoresistor 32, and is preferably selected to be approximately45°. By virtue of the characteristic distribution of light intensitiesover the detectible range of scattering angles, a corona or halo effect,illustrated by an imaginary annular region 44, is observable on thesurface of container 33.

Baffle 30 which is disposed in space 41 on the optical axis, extendsradially thereabout and prevents all light directly transmitted fromlamp 24 which passes through the region 46 from reaching thephotoresistor 32. Thus, no directly transmitted light is permitted toreach the photoresistor 32. The baffle 30 and the photoresistor 32 aresized and positioned so that light projected from the annular region 44is permitted to reach the light sensitive area of the photoresistor 32,i.e. parts of the sensitive area of the photoresistor are in the fieldof view of the halo (FIG. 4). Nothing is disposed in space 41 exceptbaffle 30 and therefore the light from annular region 44 is directlytransmitted through space 41 to the parts of the photoresistor in thefield of view 48 of the annular region 44. The light in the annularregion 44 which thus reaches the photoresistor 42 is a measure of theparticle count of the barium sulphate or of bacteria in container 33.The intensity of the light in the annular region 44 is directlyproportional to the number of particles or bacteria in the container 33,i.e. the higher the concentration of particles or bacteria, the morelight that is reflected to the annular region 44. The conductivity ofphotoresistor 32 is directly proportional to the intensity of the lightwhich impinges upon its light sensitive surface area.

With particular reference to FIGS. 2 and 3, a generally rectangularbaffle 30 is employed with a cylindrical container, such as container33, to eliminate lensing effects of the curved surface of container 33.That is, light in beam 39 which is refracted at the surface of container39 but not scattered by particles in the sample will be prevented fromreaching photoresistor 32 by baffle 30. By virtue of the rectangularshape of baffle 30, light in the upper region 44A and the lower region44B of region 44 is permitted to reach photoresistor 32.

Referring now to FIG. 5, the photoresistor 32 is shown connected in ameasuring circuit 60. The pressure sensitive switch 36 is in an on/offswitch for the apparatus and connects and disconnects AC power to theapparatus and to the circuit 60. Lines 62 and 63 connect AC power to theswitch. Depressing container 33 into receptacle 34 activates thepressure sensitive switch 36 to switch power into the circuit 60 andilluminate lamp 24 which is connected between the switched AC power line62A and the neutral AC line 63. Photoresistor 32 is disposed to receivethe light transmitted by lamp 24 and scattered by the McFarland standardconcentration or the bacteria sample in container 33 into the annularhalo region 44, as described above.

Circuit 60 in addition to Optics Section 22 includes a Rectifier Section68, a Regulator Section 70, a Calibrate Section 72 and a "Zero" Section74. The Rectifier Section 68 is connected between the switched AC line62A and the neutral AC line 63 and comprises resistor R1 connected inseries with a rectifier diode D1, and a filter capacitor C1 connected inshunt with the series-connected resistor R1 and diode D1, and theneutral line 63. Rectifier Section 68 operates in conventional fashionto provide half-wave rectifier DC at the output 76 of the RectifierSection. The half-wave rectified DC is fed to the Regulator Section 70which comprises a limiting resistor R2 connected at one terminal thereofto diode D1, and a zener diode ZD1 connected in shunt to the otherterminal of resistor R2 and neutral line 63. The Regulator Section 70operates in conventional fashion to provide a regulated DC voltage atits output 78 having a value of approximately the zener breakdownvoltage of zener diode D1. The regulated DC voltage is fed toseries-connected resistors R3 and R4 which are connected across theoutput 78 of the Regulator Section. Resistor R4 is a potentiometer whosesetting determines the DC voltage at the wiper arm 80 of potentiometerR4. Resistors R3 and R4 are connected as a voltage divider to provide adivided DC voltage at point 82. Resistor R4 is adjusted as will bedescribed below to provide a calibrated output reading on meter 65. Thewiper arm 80 of the potentiometer R4 is connected to one terminal 83 ofthe photoresistor 32 and the other terminal 84 of the photoresistor 32is connected to one terminal 85 of meter 65. The other terminal 86 ofmeter 65 is connected to the neutral line 63.

The current which flows through meter 65 is determined by theconductivity of photoresistor 32 and the setting of potentiometer R4,and by the Zero Section 74. The Zero Section comprises potentiometer R6connected across the switched AC line 62A and the neutral AC line 63, adiode D2 connected to the wiper arm 87 of potentiometer R6 and toresistor R7 which is connected in series with diode D2 and terminal 85of meter 65. Resistor R7 and diode D2 provide a negative half-waverectified voltage which is coupled to meter 65. The value of thehalf-wave rectified DC voltage is determined by adjustment ofpotentiometer R6.

Referring now to FIG. 6, another embodiment of an optical section 22A isillustrated. A lamp 90 and a negative lens 92 cooperate to provide adiverging beam of light 39 which is projected towards the container 33.An opaque baffle 96 is interposed between the lens 92 and the container33 on the optical axis 38. The baffle 96 is sized and spaced between thecontainer and lens to permit a portion 39A of the diverging beam oflight 39 to reach the container. Light 39B propagating towards thecontainer about the axis 38 is blocked by the baffle 96 and preventedfrom reaching the container 33. The diverging light 39A which enters thesample is scattered in a manner similar to that described for theembodiment of FIGS. 1-4 producing the halo region 44. The light in thehalo region 44 is permitted to reach the photoresistor 32, while thelight 39B propagating about the about axis 38 is blocked and accordinglydoes not reach the photoresistor 32. As for the embodiment of FIGS. 1-4,the intensity of the light in the annular region 44 will be directlyproportional to the number of particles or bacteria in the container 33.

OPERATION

The Standardized Inoculum Reader 10 is calibrated at the factory or inthe field as follows. A container 33 holding a "zero" sample, i.e. asample having no particles or a negligible number of particles and whichwill not produce the halo 44, is inserted into receptacle 34 and isdepressed to activate the system. Photoresistor 32 receives no light andits conductivity is therefore at a minimum. Potentiometer R6 which isaccessible from the exterior of housing 35 (FIG. 1) is adjusted toprovide a meter reading which coincides with line 18A at the extremeleft of the minus area of the meter face. Line 18A indicates a "zero"particle count. The "zero" sample container is removed and a container33 holding a McFarland standard concentration sample is inserted inreceptacle 34 and depressed to activate the system. Potentiometer R4(accessible from the exterior of housing 35 (FIG. 1)) is adjusted toprovide a meter reading coinciding with line 20 in the central region 14of the meter. The McFarland standard concentration sample container isremoved and the "zero" sample container is reinserted and thepotentiometer R6 readjusted to provide the "zero" indication. The "zero"sample container is again removed and the McFarland standardconcentration container is again inserted and the potentiometer R4 againadjusted for a meter reading coinciding with line 20. Since there issome interaction between potentiometer R4 and potentiometer R6, the lasttwo steps may be repeated until no further adjustment is necessary.

Standard Inoculum Reader 10 is now factory calibrated to comparebacteria counts to the particle count of a McFarland standardconcentration. In use, however, it may be necessary to zero andcalibrate the system using "zero" and McFarland standard concentrationsand adjusting potentiometers R6 and R4 to insure continued accuracy.

After being calibrated as described above, the Standard Inoculum Reader10 is used to determine whether bacteria samples have a predeterminedbacteria count and hence may be used as standardized samples. These are"go", "no-go" tests. The container 33 containing the bacteria sample tobe tested is inserted into the Reader 10 and depressed to activate thepressure sensitive switch 36 to activate the system. The intensity ofthe light in the halo or annular region 44 is directly proportional tothe bacteria count of the sample, i.e. the more intense the light inhalo region 44 is, the higher the number of bacteria in the sample.Photoresistor 32 has a conductivity which is directly proportional tothe intensity of light received on its light effective surface area andthe more conductive that photoresistor 32 is, the more current that isfed to meter 65. Thus, the more intense the light in the annular region44 is, the more current is supplied to meter 65. Accordingly, higherbacteria counts will read in the right region 16 of the meter whilelower bacteria counts will read in the left region 18 of that meter.Thus, if the meter reading is between the "+" and "-" area of the meter,i.e. in the central region 14, then the bacteria sample containsapproximately the predetermined number or count of bacteria and can beused as a standardized concentration. If the meter reads in the "-" area18, then the number or count of bacteria is lower than the predeterminednumber and if the meter reads in the "+" region 16 to the right of thecentral region, then the bacteria count of the sample exceeds thepredetermined number.

While the baffles 30 and 96 have been shown spaced from the container33, it is contemplated that they may be placed elsewhere in accordancewith the optical geometry of a particular system, for example on acontainer surface. Additionally, it is contemplated that optical systemsother than the lamp 24 and baffle 26, and the lamp 90 and lens 92, maybe utilized to provide a diverging beam of light 39.

The advantages of the present invention as well as certain changes andmodifications of the disclosed embodiments thereof will be readilyapparent to those skilled in the art. It is the applicants' intention tocover by their claims all those changes and modifications which can bemade to the embodiment of the invention herein chosen for the purposesof the disclosure without departing from the spirit and scope of theinvention.

What is claimed is:
 1. Nephelometer apparatus for analyzing a fluidsample of particulate material disposed in a container comprising:lightsource means for projecting light about an optical axis, the projectedlight diverging from the optical axis; photoelectric means spaced fromthe light source means and disposed on or adjacent said optical axis forreceiving light from the light source means, the electricalcharacteristic of the photoelectric means being dependent upon theintensity of light received by the photoelectric means; means forpositioning the container at a predetermined location intersecting thediverging projected light and spaced apart from the light source meansand the photoelectric means; first baffle means opaque to the light fromthe light source means disposed adjacent one of the opposite sides ofthe positioning means and intersecting said optical axis and extendingtherefrom for preventing light in a first predetermined region extendingabout the optical axis and including any direct image of the lightsource means from reaching the photoelectric means while permittinglight emerging from the container in the positioning means in a secondpredetermined region extending beyond said first predetermined region toreach the photoelectric means; the light source means being operative toproject light divering sufficiently from the optical axis to illuminateparticulate material positioned in said container to redirect saidilluminating light directly into said second predetermined region toreach said photoelectric means; and indicator means coupled to saidphotoelectric means for proportionally indicating the intensity of thelight receiving by the photoelectric means in accordance with theelectrical characteristics of the photoelectric means.
 2. The apparatusas recited in claim 1, wherein the light source means comprises a lightsource and a second baffle means opaque to the light source disposedintermediate the positioning means and the light source, said secondbaffle means having an orifice therein aligned with the optical axis. 3.The apparatus as recited in claim 2, wherein the area of said orifice issubstantially smaller than the area of the light source from which thelight is projected.
 4. The apparatus as recited in claim 3, wherein theorifice is beveled on the side thereof opposite to the light source. 5.The apparatus as recited in claim 1, wherein the first baffle means isdisposed intermediate the positioning means and the photoelectric means,whereby the light in the first predetermined region and including anyoptical image of the light source means emerging from the container isprevented from reaching the photoelectric means.
 6. The apparatus asrecited in claim 1 or 5, wherein the first baffle means comprises anelongated member disposed in a transverse direction with respect to theoptical axis.
 7. The apparatus as recited in claim 6, wherein thepositioning means is disposed in the apparatus to receive acylindrically shaped sample container having a cylinder axis disposedtransverse to the optical axis, and the elongated member is disposedtransverse to the cylinder axis.
 8. The apparatus as recited in claim 1,wherein the photoelectric means comprises a photoresistor.
 9. Theapparatus as recited in claim 1, wherein the indicator means comprises ameter.
 10. A nephelometer method for analyzing a sample containing anunknown count of particles therein comprising the steps of:projecting abeam of light about an optical axis towards the sample, the projectedlight diverging from the optical axis; permitting light emerging fromthe sample in a predetermined region spaced from and extending about theoptical axis to be detected at a location spaced from the sample;preventing light propagating along the optical axis and in an adjacentregion extending about the axis from being detected at said location;the projected light diverging sufficiently from the optical axis toilluminate particles positioned in said sample to redirect saidilluminating light directly into said second predetermined region to bedetected at aid location; and providing an indication of the count ofthe sample proportional to the amount of light detected.
 11. The methodas recited in claim 10, wherein the light propagating along the opticalaxis and in the adjacent region is prevented from being detected aftersuch light emerges from the sample.